1. Field of the Invention
The invention relates to communication of data between a transmitter and a receiver. It is particularly applicable to communication systems where digital data is transmitted over a time-variant or frequency-variant channel, such as in mobile communication systems or satellite communication.
2. Description of the Related Art
As real transmission channels distort the modulated signal by phase shift and attenuation, and as they add noise to the signal, errors occur in the received data after demodulation. The probability for errors usually rises with rising data rate, that is with rising number of modulation states and falling symbol duration. To cope with such errors, redundancy can be added to the data, which allows to recognise and to correct erroneous data. A more economic approach is the assessment of channel properties and the adaptation of coding and/or modulation schemes to the channel properties.
“Adaptive Bit Loading” algorithms assign a modulation scheme to a resource by evaluating the channel state information to determine which modulation scheme is most efficient in terms of spectral efficiency of the transmission. For example a channel that is in a deep fade state is very vulnerable against noise errors, so a very robust modulation scheme is assigned that carries only very few data bits (for example BPSK or QPSK); on the other hand a channel that is amplifying the signal is very robust against noise errors, so a spectrally efficient modulation scheme may be assigned that carries many data bits (for example 16-QAM or 64-QAM). This is related to the technique “Adaptive Modulation and Coding” described below.
“Decision-Feedback Demodulation” is an iterative process where a first rough channel estimate (or none at all) is used to demodulate the data symbols. After demodulation, and preferably after decoding, the obtained information is fed back to the channel estimator for an improved estimation resulting from the data symbols. It should be apparent that this process causes not only delay and requires a lot of computations in each iteration step, but it also depends greatly on the quality of the first rough channel estimate due to the feedback loop.
Such procedure is known for example from Lutz H.-J. Lampe and Robert Schober, “Iterative Decision-Feedback Differential Demodulation of Bit-Interleaved Coded MDPSK for Flat Rayleigh Fading Channels”, IEEE Transactions on Communications. Vol. 49, No. 7, pp. 1176-1184, July 2001.
“Adaptive Modulation and Coding” (AMC) changes the coding and modulation scheme that is employed to convey data from transmitter to receiver. The adaptation is preferably based upon one or more of the criteria channel state, required bit error rate and required data transmission rate. In communication systems the transmission is usually based upon block or frame transmission, where for example the channel state is a value which is obtained for each such frame. Consequently the adaptation rate is limited by the granularity and rate of such channel state information. Obviously the adaptation rate cannot exceed the channel state information rate, which is usually available once per frame.
A major difference between wired communication systems and wireless communication systems is the behaviour of the physical channel over which information is transmitted. The wireless or mobile channel is by its very nature variant over time and/or frequency. For a good performance in most modern mobile communication systems a demodulation of data symbols in a receiver requires an accurate estimation of the channel (also known as channel state information), usually measured by a channel coefficient, which includes knowledge about the power, the phase, or both properties of the channel. To facilitate this, usually some sort of pilot symbols are inserted into the data symbol stream which have a predetermined unambiguous amplitude and/or phase value, which can be used to determine the channel coefficient.
Usually the data symbols themselves cannot be accurately used for channel estimation, since the amplitude and/or phase are not known a priori to demodulation. This behaviour can be seen from FIG. 1 and is further detailed in Table 1 to show the number of ambiguities involved in different digital modulation schemes.
TABLE 1Properties of selected digital modulation methodsBits perModulation SchemeSymbolAmplitude AmbiguityPhase AmbiguityBPSK1None/1 Level2 LevelsQPSK2None/1 Level4 Levels8-PSK3None/1 Level8 Levels2-ASK/4-PSK32 Levels4 Levels4-ASK/2-PSK34 Levels2 Levels8-ASK38 LevelsNone/1 Level16-PSK4None/1 Level16 Levels 16-QAM43 Levels12 Levels 4-ASK/4-PSK44 Levels4 Levels64-QAM69 Levels52 Levels 
From Table 1 it follows also easily that the performance of an iterative decision-feedback demodulation scheme will further depend greatly on the number of ambiguities involved in the modulation scheme. A wrong assumption about the sent symbol leads to a wrong result of the channel estimation. Especially in modulation schemes with a high number of modulation states there is a high probability of erroneous symbols due to inevitable noise. A wrong channel estimation, in turn, leads to wrong channel correction and consequently more errors in received symbols. Therefore there is a need in the related art for improved reliability of the channel estimation.
Generally a data modulation scheme can be used well for amplitude estimation if it shows no or very few ambiguities in its amplitude levels. From Table 1 it follows that the most interesting modulation schemes are BPSK, QPSK, 8-PSK, 16-PSK, or in fact any other pure PSK scheme, since all of these use a fixed amplitude for their transmission. A scheme like 2-ASK/4-PSK might still be applicable, as an estimator “only” has to make a kind of “blind” decision between two possibilities.
On the other hand a data modulation scheme can be used well for phase estimation if it shows no or very few ambiguities in its phase levels. From Table 1 it follows that the most interesting modulation schemes are pure ASK schemes such as the mentioned 8-ASK, since all of these use the same phase angle for their transmission. Schemes like BPSK or 4-ASK/2-PSK may be applicable, as the number of two phase levels may still be reasonably low for an estimator to extract information about the channel phase angle.
Using high order modulation constellation for the transmission of data symbols, a large number of amplitude/phase ambiguities are involved. Consequently a receiver cannot easily use these data symbols to improve its channel estimation accuracy, or it requires a huge mathematical and processing capacity in order to do so. The target of the present invention is a concept for transmission of data symbols for which it is easier for the receiver to extract information about the channel.
US 2004/0128605 A1 improves the channel estimation capability of an OFDM system in a high velocity environment by replacing selected data symbols with pilot symbols. This method uses pre-determined pilot symbols which cannot carry information and therefore reduce the efficiency of the data transmission.
EP1083719 A2 claims the adaptation of the proportion of reference (i.e. pilot) symbols into a data stream in such a fashion that the overall transmission operates at improved efficiency, for example depending on the packet size. This document focuses on the distribution or the interval of such reference symbol addition. Moreover a method is needed which allows more economic use of transmission capacity.
WO 9909720 A1 introduces a rotationally invariant modulation encoder, inserting pilot bits into a data stream, involving coding of two different data streams. The receiver demodulates and decodes the data streams in an iterative fashion to use that pilot bit information for improved channel estimation. There is a need for a more simple method which does not require to add pilot bits into the user data stream nor does require a bit evaluation at the receiver.
Jie Zhu; Wookwon Lee: “Channel estimation with power-controlled pilot symbols and decision-directed reference symbols”, Vehicular Technology Conference, 2003. VTC 2003-Fall. 2003 IEEE 58th, Volume: 2, 6-9 Oct. 2003 Pages: 1268-1272 Vol. 2, introduces a least-squares scheme for decision-directed virtual pilot channel estimation. Virtual pilots are defined as data symbols which are in close proximity to their estimated transmitted value. The whole procedure is a multi-step approach, where in the first step only pilot symbols are used for a first CSI estimate. From this, data symbols are tentatively demodulated and re-modulated onto symbols. If the difference between a received and re-modulated symbol is below a threshold, that symbol is defined as a virtual pilot symbol. These virtual pilot symbols are subsequently used for an updated CSI estimation. This prior art is a receiver-specific algorithm. A method is needed which allows a receiver to extract information without the need for demodulation and re-modulation.
Marc C. Necker; Gordon L. Stüber: “Totally Blind Channel Estimation for OFDM on Fast Varying Mobile Radio Channels”, IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, Vol. 3, No. 5, September 2004, proposes a blind estimation technique which does not require pilot symbols. To resolve phase ambiguities of the estimation, a scheme is introduced which puts e.g. QPSK on one sub-carrier and 3-PSK or 5-PSK on an adjacent sub-carrier. The combination facilitates an unambiguous estimation of the phase. However a method is needed which allows to use known simple modulation schemes.